Black reflections with Cycle's Glossy Material
I'm not completely sure what's causing this, I suspect it is just how Cycles works and is not strictly a bug - similar to the terminator issue.

In order to work around it, we can modify the surface normals slightly in the material:



From the manual:


  Incoming
  
  Vector pointing towards the point the shading point is being viewed from.


Thus adding a tiny bit of the Incoming vector will skew the surface normal just enough to hide the artifacts.

Note that changing the surface normal means our reflections will be incorrect - so we need to change it as little as possible, otherwise the reflections of objects and clouds will be noticeably weird.